The invention relates to a hydraulic control means for controlling the application of a pressure medium to a hydraulically actuable shift member intended for an automatic automotive transmission, having a throttle in the pressure application line leading to the shift member and a control valve controlling an unthrottled connection bypassing the throttle between a pressure medium supply line and the shift member as a function of the application pressure prevailing ahead of the shift member in such manner that upon commencement of the application of pressure to the shift member, the connection is opened, and interrupted when a preassigned application pressure is reached, and having a device for at least temporary maintenance of an engagement pressure lying below the full application pressure and remaining at a substantially constant level.
Such a control means is known for example from GLD Pat. No. 2,502,347. In that known control means, the device provided for maintaining a substantially constant engagement pressure, below the full application pressure, is a conventional accumulator whose spring-loaded piston is subjected to the application pressure acting on the shift member by way of a branch of the pressure application line, which branches off immediately ahead of the shift member. This accumulator is to ensure that during the engagement process, at least for a certain period, during which the piston of the accumulator is forced back against spring action, an approximately constant engagement pressure is maintained which is below the full application pressure delivered by the source of the pressure medium.
Other known control means, instead of a spring-loaded accumulator, may comprise embodiments in which the accumulator piston is acted upon against the pressure of the application pressure line on one or more countersurfaces by counterpressures capable of being added as required.
In all of these previously known embodiments, however, the accumulator has always been connected directly in parallel with the shift member, so that its piston would already be set in motion when, at the beginning of the shift operation, direct unthrottled supply of pressure medium was at first meant to overcome only the slack of the shift member or its actuating means. The pressure control effect of the accumulator, on the other hand, is required only for the actual operation of applying the shift member. A premature start of the accumulator piston therefore unnecessarily reduces the adjustment travel afterwards required.